Capacitor with impregnated
polypropylene dielectric

ABSTRACT

NON-POROUS (POLYOLEFIN MATERIALS) POLYPROPYLENE FILMS ARE UNIFORMLY OR ESSENTIALLY COMPLETELY IMPREGNATED WITH A CHLORINATED (HYDROCARBON) DIPHENYL DIELECTRIC LIQUID IMPREGNANT TO PROVIDE AN IMPROVED DIELECTRIC SYSTEM PARTICULARLY APPLICABLE FOR CAPACITORS.

E. B. COX

Nov. 27, 1973 IIAIACITOH WITH IMIIH'IUNA'VF) OLYPROIYLENG DIELEC'IHECOriginal Fi led Oct.

United States Patent O 27,824 CAPACITOR WITH IMPREGNATED POLYPROPYLENEDIELECTRIC Eugene B. Cox, Fairfield, Conn., assignor to General ElectricCompany Original No. 3,363,156, dated Jan. 9, 1968, Ser. No. 587,835,Oct. 19, 1966, which is a continuation-impart of Ser. No. 508,529, Nov.18, 1965, and Ser. No. 513,240, Dec. 13, 1965, both now abandoned.Application for reissue Apr. 12, 1971, Ser. No. 135,505

Int. Cl. H01g 3/175. 3/195 U.S. Cl. 317-259 14 Claims Matter enclosed inheavy brackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT F THE DISCLOSURE Non-porous [polyolein materials] polypropylenefilms are uniformly or essentially completely impregnated with achlorinated [hydrocarbon] diphenyl dielectric liquid impregnant toprovide an improved dielectric system particularly applicable forcapacitors.

This application is a continuation-in part of copending applications,Ser. Nos. 508,529 and 513,240, filed Nov. 18, 1965, [both now abandoned]and Dec. 13, 1965 respectively, both now abandoned, and both assigned tothe same assignee as the present invention.

The present invention relates to improved impregnated synthetic resindielectric systems having long life, high voltage stresscharacteristics, and more particularly to A-C electrical capacitors ofthe foregoing type utilizing an impregnated [polyolefn] polypropyleneresin delectric system.

The advent of more complex and sophisticated electrical apparatus andthe trend to higher operating etliciency of present apparatus has led tomore stringent requirements for the capacitor elements of suchapparatus. For example, there is an indicated need for higher powerrated capacitors of greater efficiency, smaller size and cost. Capacitorelements, particularly A-C capacitors having significantly higherdielectric strength and corona start or extinction voltages, are mostdesirable in order to alleviate substantial problems in electricalapparatus design and operation and to improve operation of existingapparatus.

It is an object of this invention to provide an improved capacitorhaving substantially increased volumetric efficiency.

Another object of the present invention is to provide an electricalcapacitor having a solid synthetic resin dielectric spacer ofsubstantially reduced thickness but which is capable of withstandinghigh voltage alternating current stress.

A still further object of this invention is to provide an improved A-Celectrical capacitor having a [polyolefin] polypropylene tilm and ahalogenated [hydrocarbon] dphenyl impregnant as the major component ofthe capacitors dielectric spacer.

It is yet lanother object of this invention to provide an improved highvoltage A-C capacitor including a polypropylene film impregnated withtrichlorodiphenyl.

1 have discovered that certain prescribed combinations ice of materialsand processing will provide an impregnated synthetic resin capacitor ofunexpectedly favorable electrical characteristics. In one preferred formof this invention a synthetic [polyolefin] polypropylene resin, [forexample a polypropylene,] is impregnated with a halogenated [aromaticmateria1,] dphenyl for example trichlorodiphenyl, and utilized as thedielectric spacer member in a capacitor element. It has been furtherdiscovered that the mentioned materials combine and coact in such amanner to provide impregnation of the [polyolem] polypropylene of a kindwhich, in combination with other characteristics of the materials,signilicantly improves the most important electrical criteria ofcapacitors, such as dielectric strength, corona start and extinctionvoltage, long life under voltage stress, and low power factor.

While this specication concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description taken in conjunction with the acompanying drawingsin which:

FIG. 1 is an enlarged cross sectional view of a portion of anessentially completely impregnated resinous dielectric spacer;

FIG. 2 is a perspective view of a partially uncoiled convolutely woundcapacitor;

FIG. 3 shows a full assembled capacitor which includes a capacitor ofthe type shown in FIG. 2 and a container therefor;

FIG. 4 is an elevational cross sectional view of one portion of anelectrical capacitor having an impregnated resinous film as a componentof the dielectric spacer and denoted as a full sandwich structure;

FIG. 5 is an elevational cross sectional view of a portion of anotherelectrical capacitor having multiple impregnated resinous films ascomponents of the dielectric spacer and denoted as an inverted sandwich;

FIG. 6 is an elevational cross sectional view of a portion of stillanother electrical capacitor which includes a relatively thickimpregnated resinous film in its dielectric spacer and denoted as asemi-sandwich structure;

FIG. 7 is an elevational cross sectional view of a portion of a modifiedsemi-sandwich capacitor having multiple adjacent impregnated resinousfilms as components of the dielectric spacer, and

FIG. 8 is an expanded cross sectional view of a portion of an electricalcapacitor in which the dielectric spacer comprises only impregnatedresinous films.

Referring now to FIG. 1, there is illustrated one preferred dielectricspacer embodiment 10 of this invention including a section of a solid[polyolen] polypropylene resin dielectric material 11 having numerousminute discontinuities 12 otherwise characterized as apertures, voids,and interstices, whose presence is a recognized characteristic of theresin. This material is described as nonporous in that there are few ifany pores or passages interconnecting and passing through side surfaceswhich would permit the movement of an impregnant fluid of this inventionthrough opposite side surfaces. The [polyoleiin] polypropylene resinmaterial is impregnated with .a dielectric liquid impregnant whichpermeates the material itself as well as filling these discontinuities,and the composite constitutes a continuous, though heterogeneous,dielectric system. Surprisingly, the kind of impregnation obtained bythe teachings of this invention in combination with specific materialsleads to a kind of synergistic effect the result of which increases thedielectric strength of the combination. In one respect impregnationincreases the [electrical insulating] dielectric qualities of the resindielectric by incorporating in the resin an impregnant material having ahigher dielectric strength than the air or gas in the describeddiscontinuities. Examples of capacitors utilizing other impregnateddielectric systems than those herein are found in U.'S. Patents2,864,982 Ruscetta and 2,307,488 Clark, each assigned to the sameassignee as the present invention.

Unexpectedly good results have been obtained from the composite of FIG.1 when utilized for example as a capacitor element, particularly for ahigh voltage A-C capacitor, where certain combinations of [polyolefinmaterials] polypropylene and impregnants are utilized. While numerouscombinations of materials other than the [polyolefins] polypropylene andimpregnants of my invention have been described heretofore, theprior-known combinations have been found to be unproductive of thedesirable characteristics and results demanded by the sophisticated andcomplex electrical apparatus now being employed. [Those materials whichprovide the best results in this invention are the synthetic resinstaken from the class of resins known as the polyolefins, andspecifically polypropylene, polyethylene, 4-methyl pentene-l, andpolystyrene] The generally favorable characteristics of [the polyolefinsare their] polypropylene are its electrical properties as an ordinarydielectric[s] (other than for capacitor elements) good temperaturestability, and mechanical properties such as case of working andforming, particularly into thin films.

While these favorable characteristics are found in numerous uses,applicability to dielectric systems such as capacitors has been limitednotably because of low dielectric strength, low corona start voltage,and relatively short life under high voltage stress. Dielectric strengthis most important and is a measure of the ability of the material towithstand voltage stress, i.e., voltage differential across adimensioned unit of thickness. Corona start voltage (CSV) and coronaextinction voltage (CEV) are the voltages at which deleterious coronadischarge may commence and be extinguished, respectively. At the sametime, a number of problems associated with impregnation, degree ofimpregnation and compability with impregnants have been deterrents toany impregnant method of increasing the desirable electricalcharacteristics of `[polyolefn material] polypropylene. I havediscovered that [the polyolens, and particularly] polypropylene.[,] maybe impregnated to an unexpectedly high degree with halogenated[aromatic] diphenyl liquids, and when so impregnated these elementscombine and coact to provide the improved dielectric spacer of thisinvention. A preferred material [of this class of polyolefins] ispolypropylene resin, particularly an isotactic biaxially orientedpolypropylene film, a more complete description of one example of whichmay be found in Applied Plastics, November 1961, pp. 3S through 64, andModern Dielectric Materials, J. B. Birk, London, Heywood and Co., 1960,pp. l40-l42, incorporated by reference herewith.

The [polyolefin material] polypropylene of the mentioned articles [maybe] is described as linear, regular head to tail polymers of[unsaturated hydrocarbons] propylene of the formula CH2=C[HR]H3, [i.e.,alpha olefins, where R is a saturated aliphatic, an alicyclic, or anaromatic radical] copolymers of said [unsaturated hydrocarbons]propylene with one another, and copolymers of the [unsaturatedhydrocarbons] propylene with at least one other monomer copolymerizabletherewith. This [polyolen material] polypropylene may also be referredto as non-porous in that there are essentially no continuous passagestherethrough which would pass the preferred liquid impregnants of thisinvention under presently known capacitor operating conditions.

A preferred impregnant, according to my invention, is a halogenated[organic material] diphenyl being generally a compound having from 1 to5 halogen substituents such as chlorine, and from 1 to 3 aryl groups.More specifically, a preferred impregnant is trichlorodiphenyl andcommercially available as Pyranol 1499.1 This material has a high coronastart and corona extinction voltage.

The combination of Pyranol 1499 dielectric liquid with a non-porouspolypropylene film as an impregnated dielectric system provides the bestresults according to this invention. It has been indicated that thesematerials have been considered incompatible or undesirable and hence tobe avoided for dielectric system applications because polypropyleneshows evidence of being readily dissolved f by halogenated [organiccompounds] diphenyls such aS Pyranol 1499 liquid, and also of not beingwettable by the mentioned liquid impregnants. It has also been indicatedthat the dissolving of polypropylene in a non-polar liquid leads toplasticizing effects such as swelling and low tensile strength. I havefound, however, that except at very high temperatures, above about 100C., polypropylene is soluble in halogenated [aromatic materials]diphenyls only to a limited extent, and also that the partial solubilitysurprisingly does not adversely affect capacitor characteristics. Thispartial solubility of polypropylene film in trichlorodiphenyl undercontrolled temperature conditions of less than about 100 C. actuallyproves to be an important characteristic of this preferred combinationin the present invention. In particular, the partial solubility whichtests indicate to occur after initial penetration of the tilm, aids themigration of impregnant into the film as well as into the intersticesand voids thereof. This enhancement of impregnation is evidenced by anextremely high corona start voltage in the impregnated system, even inthe absence of a wick or porous layer on both sides of the film.

Representative samples of Pyranol dielectric impregnated polypropylenefilm subjected to electrical capacitor tests indicate a closecorrelation between the kind and degree of impregnation and corona startvoltage, and uniform or essentially full and complete impregnation is animportant feature of this invention. The combination of polypropyleneand Pyranol dielectric liquid is favorable to a kind of impregnation,[which in] in which the polypropylene is permeated or penetrated anduniformly impregnated by the Pyranol dielectric liquid. In the presentinvention the result of this kind of impregnation, carried out under theparameters of this invention, is defined as essentially completeimpregnation. When the voids and interstices of the material areessentially filled with liquid impregnant and the impregnation processincludes both adsorption of the impregnant liquid by the material itselfas well as partial solution of the material in the liquid impregnant,the material is referred to as essentially or completely impregnated.Representative tests with specific resin systems with varying degrees ofimpregnation indicate very high and consistent CSVs, near the measured,computed, or ultimate CSV of the system, with systems where theimpregnation process was extended or otherwise aided for essentiallycomplete impregnation. An impregnation process which is an example ofessentially complete impregnation involves submerging polypropylene filmin Pyranol 1499 at a temperature of about C. Under these conditions,relatively stable conditions are reached in about 6 to 20 days whereinabout 1.0% by weight of polypropylene is dissolved in the Pyranol liquidand about 11% by weight of Pyranol liquid is taken up by thepolypropylene. The kind and degree of impregnation may also be measuredby the CSV of the system, values approaching a maximum indicatingessentially complete impregnation.

The preferred polypropylene films used in the present inventiongenerally comprise isotactic polypropylene. This is a high molecularweight stereo-regular crystalline ma- 1 Trademark of the GeneralElectric Company.

terial including, in addition to the crystalline phase whichpredominates, an amorphous or non-crystalline phase. In somecommercially available isotactic polypropylenes, the amorphous phase maycomprise as much as 30% of the total resin. Films of these resins may-be formed, for purposes of the present invention, for example byrolling, extruding, pressing, solvent-casting, and melt casting. Inorder to improve the mechanical properties of resinous films, it iscommon to impart some form of ordered structure to such films bystretching and heat setting. IPreferably stretching is done in mutuallyperpendicular directions, i.e., both longitudinally and transversely ofthe film length, so as to impart a biaxial orientation to the film.Films may also be uniaxially oriented, biaxially oriented, balancedbiaxially oriented. [Polyolefin iilrns, particularly] The polypropylenefilms[,] should have little if any residual contaminants present thereinwhich might impart a high power factor, i.e., the measure of powerdissipation in a given material, to the composite. Contaminants also maybe external materials taken up from film processing operations orresidual catalysts. These contaminants may be removed by appropriatesolution of the [polyolefin] polypropylene and removal of contaminantsby precipitation extraction and adsorption methods. However, excellentresults have been obtained with the use of commercially availablepolypropylene resins such as for example Profax 6520F Resin (HerculesPowder Co.) and Shell SSUUF Resin (Shell Oil Co.)

Capacitor devices embodying the present invention, such as thoseillustrated in FIGS. 2 and 3, may have the same general configuration ofpresently known capacitors. Referring now to FIG. 2, there is shown aconvolutely wound capacitor 14 comprising separate electrode foils orarmatures 15 and 16, and intermediate dielectric spacers 17 and 18.Terminal connectors 19 and 20 have enlarged surfaces 21 and 22 (notshown) in contact with electrode foils 1S and 16. Electrode foils 15 and16 may comprise one or more of a number of different materials,generally metallic, including for example aluminum, copper and tantalum.Dielectric spacers 17 and 18 generally comprise a composite or sandwichtype structure which includes at least one impregnated resinous layer 11in accordance with the present invention. More specifically, adielectric spacer 17 and the metallic electrode foils 15 and 16, takentogether, comprise a capacitor element structure.

Referring now to FIG. 3, there is shown an assembled capacitor unit 23in which is encased a convolutely wound capacitor of the type shown inFIG. 2. The assembled unit also includes a container 24, a hermeticallysealed cover 25, which includes a small dielectric liquid fill hole 26,and a pair of terminals 27 and 28, projecting through cover 25 andinsulated therefrom. Within the container 24, terminals 27 and 28 areconnected to terminal connectors 19 and 20, as shown in FIG. 2. Althoughnot illustrated, the unit 23 shown in FIG. 3 further includes adielectric liquid which occupies the remaining space in container 24 notoccupied by the capacitor element, and which also impregnates thedielectric spacers 17 and 18 shown in FIG. 2.

In one general impregnation method, as described in the mentioned Clarkand Ruscetta patents, capacitor units or encased assemblies such ascapacitor 23, FIG. 3, of the present invention, are usually vacuum driedto remove residual moisture. The drying temperature will vary dependingon the length of the drying cycle, but usually ranges from about 60 to150 C. With too low a temperature the drying speed is excessively longwhile too high a temperature may cause decomposition of any papercomponent of the dielectric spacer. Hole 26 permits withdrawal ofmoisture from the interior of container or housing 24 during the dryingprocess.

The impregnating dielectric liquid is admitted to the capacitor assemblythrough hole 26 preferably while the dried assembly is still undervacuum in a suitable evacuated enclosure. Usually, enough oftheimpregnating liquid is introduced to at least submerge the capacitorelement in the container. The pressure in the enclosure is then raisedto atmospheric pressure and the assembly permitted to stand or soak fora number of hours for thorough penetration of the liquid impregnant.After impregnation, the capacitor unit may be sealed by applying aquantity of a suitable solder to hole 26. If the impregnant is apolymerizable resinous material, the capacitor assembly is thereaftersubjected to an elevated temperature to effect polymerization andsolidification of that material. In addition to the foregoing process,other techniques, which generally utilize heat and/or pressure, may beutilized in the practice of the present invention, for example, a numberof methods including cycling of pressures, temperatures, or both may beemployed to aid in the impregnation process. Heat and pressure mayenhance irnpregnability by changing the relative wettability, viscosityand solubility of the materials. In addition, expansion and contractionof individual components of the system, which may be the result of heatand pressure, may act as a driving force to induce migration of liquidinto the solid dielectric, particularly with hole 26 sealed.

Excellent results were obtained in this invention by including a heatingcycle after impregnation or sealing to aid migration of the liquid intothe solid dielectric to effectuate or ascertain higher degrees ofimpregnation or essentially complete impregnated dielectric systems,particularly capacitors. For example, wound and assembled capacitors arefirst impregnated by baking and evacuating the assembled capacitors andthen filling them with or immersing them in a body of dielectric liquidimpregnant which may be preheated or subsequently heated to enhanceimpregnation. Following this treatment, the assernbled and impregnatedcapacitors are sealed and the sealed units are subjected to elevatedtemperature for an extended period of time.

A preferred heating cycle in the practice of this invention is definedas a controlled heating period utilizing elevated temperatures in the 65to 95 C. range for a period of from about 4 to about 16 hours.Variations in processing, utilization of pressure, and additives mayshorten the time period. A-C capacitors of high voltage utilizing apolypropylen film-paper composite dielectric spacer and a Pyranol 1499dielectric liquid impregnant were heated in a temperature range of to 95C. from 4 to 16 hours and were .found to have consistently high CSV.

The temperature conditions are controlled so that partial solution ofthe [polyole'fin] polypropylene takes place in the dielectric liquid anddielectric impregnant is dissolved in the resin to provide essentiallycomplete impregnation. Increased permeation of a polypropylene film forexample may be enhanced by the fact that some of the amorphous and/orlow molecular weight portions of the polypropylene may dissolve in theliquid at about 85 to C. More consistent and higher CSV characteristicshave been noted when capacitors of this invention are subjected to theforegoing controlled temperature irnpregnation process. Impregnation mayalso be further improved by modifying the physical characteristics ofthe components of the impregnated dielectric system. More specifically,mixtures of dielectric liquids, or additives, may be included in thedielectric liquid impregnant, or the solid dielectric material may betreated so as to effect the impregnability of the system. For example,Pyranol 1475 dielectric liquid, comprising primarily trichlorobenzene,may be added to Pyranol 1499 dielectric liquid in an amount for example25 percent by weight. Other dielectric liquids which may be effectivelyemployed in mixture with Pyranol for example are liquid mineral oil andsilicone oil.

The impregnated dielectric systems of the present invention exhibitcertain significant dielectric properties which render them highlyadvantageous for [many electrical applications including insulatedelectrical devices generally,

such as electrical cables and transformers, as well as] use incapacitors. These properties are generally in three categories, i.e.,increased dielectric strength, low energy loss in the dielectric, andhigh CSV by reason of impregnation characteristics. Impregnation is mostimportant because the kind and degree of impregnation controls the CSVattainable in the system. Increased dielectric strength is importantbecause it provides a more efficient dielectric and also permits the useof a smaller volume or weight of dielectric material to withstand agiven voltage in a system. Energy loss is important since energy loss inthe system adversely affects the electrical efficiency of the unit andmay cause physical deterioration of the structural materials of the unitdue to the conversion of the dissipated energy into heat.

Most importantly, these significantly improved properties may beincorporated in A-C capacitors of high voltage stress capabilities byutilizing the dielectric of this invention. A-C capacitors have beenassembled which are long-life operable at a voltage stress in excess ofabout L 1200 volts per mil of impregnated dielectric and a CSV fromabout 750 volts to in excess of 3000 volts. Development of high voltageA-C capacitors has been previously limited because of the relativelyshort operating life of the dielectric under high voltage stressconditions. For example, previous A-C capacitors for long-life operationare operable in the general range of voltage stresses of less than about500 volts and under short life pulse conditions may reach only about 750volts.

Other examples of capacitor elements incorporating the improveddielectric of this invention are illustrated in FIGS. 4 through 8. InFIG. 4 there is shown a portion 29 of a capacitor in which one type ofcomposite dielectric spacer, termed the full sandwich, is used. Thiscapacitor portion comprises an impregnated resinous dielectric film 11,interposed between a pair of impregnated poro-1s dielectric sheets 30and 31, and a pair of electrode foils 1S and 16. Sheets 30 and 31 may bea wel] known paper such as kraft paper and also may be suitablyimpregnated with a liquid dielectric, for example the dielectric liquidof this invention. The term porosity as applied to this paper indicatesa substantial number of continuous passages or pores in and through thepaper which are capable of passing the liquid impregnant through thepaper from one side surface through the opposite side surface. The CSVcharacteristics of an impregnated resinous film are in large partdependent on the effective impregnation of the film interstices as wellas between the surfaces of that film and any adjacent material. The CSVof a full sandwich dielectric, such as that shown in FIG. 4, isincreased by the use of an adjacent surface such as paper.

In FIG. 5 there is shown an embodiment 32 which utilizes anothercomposite dielectric spacer structure. This has been termed the invertedsandwich. This composite dielectric spacer comprises a single sheet ofimpregnated porous material 30 or 31, interposed between a pair ofimpregnated resinous films 11 and 11'. The composite dielectric spaceris itself interposed between a pair of electrode foils 1S and 16.Exemplary capacitors of this type included a single sheet of 0.66 milkraft paper for the porous material between two sheets of 0.50 milpolypropylene. This ordinarily difficult to impregnate system wasimpregnated without difficulty to yield a capacitor unit of 0.9 mf. anda CSV of greater than 2650 v. A-C root mean square.

The unique combination of polypropylene and Pyranol 1499 provides easeof impregnation even in a tightly wound roll where formerly other priorart combinations were required to rely on loosely wound rolls for fullerimpregnation. An important advantage of the polypropylene-Pyranolcombination in this as well as other structures is that thepolypropylene passes the impregnant therethrough to reach heretoforedifficult to reach voids and interstices in remote areas from the originof impregnation, and particularly those along and near the foil orelectrode to film interface.

A capacitor dielectric spacer structure 33 similar to that shown in FIG.5 is shown in FIG. 6. This structure 33, denominated the semi-sandwich,differs from the inverted sandwich by the omission of one of theimpregnated resinous films 11 or 11' therefrom.

Another modification of the present invention, as shown in a capacitorstructure, is seen in FIG. 7. This embodiment, which utilizes acomposite dielectric spacer 34 referred to herein as a modifiedsemi-sandwich, comprises two contiguous impregnated resinous films 11and 11' and an impregnated porous sheet 30 adjacent thereto. As in theother embodiments, this composite dielectric spacer is interposedbetween a pair of electrode foils 15 and 16. The purpose of placing theresinous films 11 and 11' adjacent one another is to prevent dielectricfailure due to an imperfection in a single thickness of resinous film.Adjacent resinous films tend to block any imperfection in one of thefilms and thereby prevent failures. FIG. 7 represents a much improvedcapacitor element in which a pair of adjacent films providesimpregnation characteristics more favorable in many respects than asingle film of equivalent thickness.

In FIG. 8 there is shown an electrical capacitor structure 34 having twoadjacent impregnated resinous films 11 and 11', as the dielectricspacer, interposed between a pair of electrode foils 15 and 16. Twoadjacent thin films 11. and 11' are used, rather than one film twice asthick, for the same reason pointed out with respect to the embodimentshown in FIG. 7. Another important feature of the embodiment shown inFIG. 8 is the absence of any porous sheet such as 30 or 31 (FIG. 4) toact as a wick or impregnation-facilitating layer.

It will be appreciated that in each of the embodiments shown in FIGS. 48, many minor modifications may be made. For example, metallized filmson the outer surfaces of the composite dielectric spacers may be used toserve as electrodes in lieu of the electrode foils 1S and 16illustrated. Similarly, the resinous films in each of the aboveembodiments may comprise either self-supporting films or supported filmsformed, as a coating or layer, on another element of the capacitorstructure, such as an electrode foil or porous dielectric sheet.

In the structures shown in FIGS. 5 through 8, at least one surface ofthe resinous film is adjacent a relatively non-porous surface, such asan electrode foil or another resinous film. It is very important tosufiiciently or completely impregnate a resinous surface, but difiicultto do so when the resinous film surface is adjacent a relativelynon-porous surface. However, by means of the present invention suchdifficulty is minimized. Consequently, those structures, such as thoseshown in FIGS. 4-8, having relatively high CSVs, are provided( for thefirst time) in this present invention.

To illustrate these improved properties in the dielectric systems of thepresent invention, a considerable number of capacitor elements asillustrated in FIGS. 1-8 were assembled and subjected to standardelectrical capacitor tests and service life conditions and measurementscomparisons have been made.

Synthetic resin materials are known to have extremely high intrinsic(small area) dielectric strength. For example, as used in the presentinvention the impregnated polypropylene film has operable dielectricstrength of over 1200 volts per mil, although its intrinsic dielectricstrength may be over 20,000 volts per mil, based on an area of about0.01 square inch. Impregnated paper, the most common dielectric materialpresently in use in A-C capacitors, has an operable dielectric strengthof about 400 volts per mil. The extent to which utilization of thehigher dielectric strength resinous dielectrics of this inventionreduces the quantity of dielectric material required in various types ofa given system may be illustrated by comparing test results of severalsimilarly impregnated dielectric spacers for electrical capacitors.Ihese types, which include allpaper, i.e., only paper sheets betweenelectrodes, paperpolypropylene film composites, and all film, i.e., onlypolypropylene film between electrodes, are listed in Tables I and II.

For example a 50 k.v.a.r. capacitor having an inverted sandwichpolypropylene-paper dielectric impregnated with Pyranol 1499 dielectricliquid having an epoxy compound stabilizer occupies 40% less volume thanthe prior art all- 5 paper design or, in other words, is a little overone-half TABLE I Resin traction Total (percent o! Operating thicknesstotal space voltage stress Description Composition (mils) thickness)(v./mll) All a er Three 0.3 mil paper 0.9 0 400 Fulfsaiidwlch- 0.3 milpolyproiylene film be- 0. 9 33 670 tween two D. mil paper sheets:Semi-sandwich 0.%h11n1aper+0.45 polypropylene 0. 9 50 800 Invertedsandwich 0.3 mil paper between two 0.3 mil 0. 9 67 930 polypropylenefilms. All film 0.9 mil po ypropylene tlm or two 0. 9 100 1, 200

0.45 m11 polypropylene 111m.

In Table I, a standard total dielectric thickness of about 0.9 mil isused since some of the composite spacers require three thicknesses totalof either tilm or paper and the minimum practical thickness for bothfilm and paper is about 0.3 mil. The voltage which each of thesecombinations may be expected to operate with long service life, aslisted in Table I, illustrates the advantages of using a [polyolefin]polypropylene iilm either as a supplement or as a substitute for thepaper dielectric spacers heretofore used. These values may be intluencedin various systems by the degree and kind of impregnation and by theuniformity of dielectric properties in the system. These values includean accepted approximation of the ratio of dielectric constants ofimpregnated paper and impregnated polypropylene. Specifically, a ratioof 3:1 was used. It should be noted that the distribution of voltage inthe system resulting from this ratio of dielectric constants results ina stress on the film components of about 1200 volts per mil, which is anoperable dielectric strength of polypropylene, providing long servicelife.

as large. If the k.v.a.r. capacitor of this invention were as large as aprior art 50 k.v.a.r. capacitor, the capacitor of this invention wouldhave a markedly higher capacity rating. By comparison, an all-paper unitof approximately the same size as the 50 k.v.a.r. lm composite capacitorhas a rating of approximately 30 k.v.a.r. Corresponding significantweight reduction is also achieved.

Comparing a 150 k.v.a.r. capacitor with a dyranolpolypropylene-paperdielectric to a 100 k.v.a.r. capacitor with a Pyranol-paper dielectric,the former is smaller and weighs 0.7 pound per k.v.a.r. The latter,which represents a typical prior art capacitor, weighs 1.29 pounds perk.v.a.r.

Although the overall weight and volume of electrical capacitors havingany given rating may be reduced by means of the present invention, itshould be noted also that in some instances there may be a maximumpractical size for an electrical capacitor and the present invention mayalso be used to produce capacitors of this size having TABLE II VoltageOperating Total capability Volumetrle voltage thickness of the eficiencystress Description Composition (mils) system pf. (nt/in!) (v./m11) Allpaper Three 1.0 mil 3.0 1, 200 14 400 F011 sandwich 0.0 mil ilm betweentwo 1. 8 1, 200 22 670 0.6 mil paper sheets. Semi-sandwich 0.75 millilm+0.75 mit l. 5 1, 200 26 800 paper sheet. Inverted sandwich 0.43 milpaper sheet be- 1. 29 1, 200 29 930 tween two 0.43 l films All thn One1.0 mil 01m 1.0 1,200 .36 1, 200

In Table II, the same types of dielectric spacers are listed along withthe thicknesses thereof required to withstand a total voltage on thesystem of 1200 volts. For purposes of this computation, the thicknessesof each lilm and paper sheet were considered to be the same in any givencomposite type. A still more efficient arrangement for a number ofapplications would include a minimum thickness of paper, and tocompensate for this minimum thickness slightly thicker films are used.The data in Table II are indicative of the reduced amount of dielectricmaterial required to withstand a given voltage as the proportion ofresinous material in the dielectric is increased. Table II alsoindicates the volumetric eliiciency to be expected in capacitors havingdielectric spacers of the types listed. These values are given inmicrofarad per cubic inch of dielectric spacer.

Both 50 and 150 k.v.a.r. (kilovolt ampere reactive) units, embodying thepresent invention, have been built and operated for long periods oftime, including thousands of hours of service life testing. These unitsare designed for and operated at voltage stress levels which result in astress on the resinous component of the dielectric system approaching1200 volts per mil. The size and weight of these units demonstrate thatthe potential improvements illustrated by Tables I and II have beenrealized.

a higher rating. In all of the above cases, the improved weight andvolumetric elciency of the units can be attributed to the use of acombination of materials which results in a stress level in the resinouscomponent approaching the maximum practical stress bearing capability ofthat component.

There are many applications, such as those utilizing high voltage powercapacitors, in which it is highly desirable to reduce energy dissipationin the dielectric system to the greatest extent possible. Theimpregnated resinous dielectric systems of the present invention areparticularly advantageous in these applications. The power factor of thesystems of this invention is generally between about 0.05 and about0.15% at rated voltage, even at substantially above room temperature.This represents a significant improvement over typical impregnated priorart systems with power factors from 0.2 to above 0.5%, and furtherpermits a reduction of as much as 40% in size over the mentioned typicalcapacitor in larger sizes.

As an example of the reduced energy dissipation in the impregnatedresinous dielectric systems of the present invention, a test wasconducted on a 50 k.v.a.r. capacitor with a `Pyranol 1499-impregnatedinverted sandwich polypropylene spacer. This capacitor, as pointed outabove, is

40% smaller than its all-paper 50 k.v.a.r. counterpart. The amount ofenergy dissipated in this capacitor was indicated by the dielectrictemperature rise, Le., the amount of temperature rise measured in thedielectric of the capacitor over ambient temperature. In this test a C.dielectric temperature rise was measured in the film capacitor ascompared to a 48 C. temperature rise in a comparison all-paper unit. Inaddition, after a 500G-hour life test at 55 to 70 C., the dissipationfactor of the inverted sandwich unit was about 0.05% as compared toabout 0.2% for the all-paper unit.

As an example of the stability of power or dissipation factor in aPyranol 1499-polypropylene dielectric system, a test was conducted on agroup of electrical capacitors having semi-sandwich dielectric spacerscomprising adjacent sheets of 0.5 mil polypropylene and 0.4 mil kraftpaper, impregnated with Pyranol 1499 dielectric liquid including about1% by weight lepoxyethy1-3,4epoxy cyclohexane. These capacitors weretemperature cycle tested and aged and the following measurements ofdissipation factor were made at the rated voltage of the capacitors, 460volts A-C, 60 cycles per second:

These results indicate the highly stabilized nature of the dissipationfactor in the system through a temperature range from 25 to 85 C., andthrough more than 5000 hours of use.

Since impregnation is important to prevent the formation of coronadischarge in a solid dielectric, the impregnation characteristics of thedielectric systems of the present invention is an importantconsideration. In some applications, such as those utilizing highvoltage power capacitors, CSVs well above 2000 volts are required. A1-though many physical characteristics of both the resinous material andthe dielectric liquid impregnant may be involved in determining theimpregnability of the overall system, permeability of the resin to theliquid is related to the solubility of the resin in the liquid. Thisrelationship has been demonstrated by a test in which a quantity ofPyranol 1499 dielectric liquid impregnant was contained in a bag orenvelope made of a non-porous polypropylene resinous film as employed inthis invention, with the envelope placed in an oven at about 75 C.Permeation, through the bottom portion of the envelope, was observed bypassing the bottom of the envelope contiguously across a microscopicslide. When the dielectric liquid had permeated the envelope, a smearwas produced upon the microscopic slide. Using this test, it has beendemonstrated that polypropylene iilrn is not permeated by Pyranol 1499dielectric liquid, after many hours, at room temperature. Permeation canbe observed after only a few hours, however, when the temperature of thesystem is raised to 75 C. and above.

When temperature impregnation is combined with a pressure application,such as an external pressure application, or an internal pressureapplication by heating essentially complete impregnation, as evidencedby CSVs consistently above 2500 volts, is attained for more ditlicult toimpregnate units. lFor example, in a large tightly wound capacitor withthe dielectric system adjacent a non-porous material, such as anelectrode metal foil, the confines of the system permit only limitedaccessibility of the liquid dielectric to the dielectric system and itis for this reason, pressure in additon to temperature, desirable toproduce optimum impregnation. It is significant that both in the bagpermeation test and in the capacitor impregnation test, with Pyranol1499 dielectric liquid, the effect of the dielectric liquid on thedielectric lilrn is substantially dilerent at room temperature than itis at temperatures in the range from to C.

To demonstrate the consistently high CSVs which may be produced in thecapacitors of the present invention, three 40 k.v.a.r. convolutely woundinverted film sandwich capacitors, each comprising a sheet of 0.3 milpaper interposed between two sheets of 0.5 mil polypropylene, wereimpregnated with Pyranol 1499 dielectric liquid, which included a smallamount of an epoxytype stabilizer. These capacitors were 10.5 incheswide and had an initial CSV of 750 to 1050 v. A-C. The units were heatedto C. for several hours and essentially complete impregnation wasattained, indicated by CSVs above 3000 volts. After testing otherelectrical characteristics of the capacitors, CSV was rechecked. Theresults of this test are summarized in Table IV.

The extremely high CSV in these capacitors, along with the consistencywith which it has been attained, is taken to be an indication thatessentially complete impregnation has `been attained. Another indicationthat essentially complete impregnation has been attained is that themeasured values of CSV app-roach the ultimate that would be expectedbased on mathematical computations.

Pyranol 1499-impregnated polypropylene-paper dielectrics aresignificantly more resistant to corona damage than are conventionalimpregnated paper dielectrics. Specifically, test capacitors, such asthe kind illustrated in FIG. 5, subjected to a 300% overvoltage (i.e.,three times their rated voltage capacity) for 30 seconds were found tohave relatively little corona damage and actually improved powerfactors. These test capacitors included a Pyranol 1499-impregnatedpolypropylene-paper dielectric. Conventional prior art paper and otherpaper resin capacitors, by comparison, tested in the same way exhibiteda substantial increase in power factor and a significant amount ofcorona damage. Corona damage was assessed in both cases by disassemblyand visual inspection of the dielectrics.

In addition to the solid dielectric materials and the dielectric liquidwith which they are impregnated, the systems of the present inventionmay also include numerous other components. In particular, it is oftendesirable to include a component to act as a stabilizer in theimpregnated dielectric system. Generally, the purpose of having astabilizer in the system is to neutralize certain contaminants orextraneous materials which may `be present or which may be formed in thesystem. Such contaminants may include residual catalyst, or catalystactivators or neutralizers, which remain from the resinforming reaction.Another source of such contaminants may include degradation productscaused by environmental or voltage-induced chemical reaction in thesystem. These undesirable contaminants and extraneous products have anadverse effect on the dissipation or power factor of the impregnateddielectric system. Stabilizing agents have been found to be highlyeffective in stabilizing the power factor of an impregnated resindielectric system.

Examples of stabilizingagents are dipentene dioxide, andl-epoxyethyl-3,4 epoxycyclohexane, which are more fully disclosed andclaimed in U.S. Patents 3,242,401, Katchman, and 3,342,402, Stahr etal., assigned to the same assignee as the present invention. Moreparticularly, l-epoxyethyl-3,ft-epoxycyclohexane has been employed inthis invention in dielectric liquids in amounts 13 in the general rangeof 0.001% by weight to about 8.0% by weight. A preferred range is about0.35% by weight to 1.0% by weight, using polypropylene iilm and aPyranol liquid impregnant.

Particulate inorganic material, such as alumina, may also be used as astabilizing agent. The electiveness of this material to correctlong-term power factor deterioration and to improve capacitor life, aswell as to improve impregnability, is more fully described and claimedin my copending application S.N. 559,030 filed May 24, 1966, now U.S.Patent 3,340,446 also assigned to the same assignee as the presentinvention.

Another component which is often used in the impregnated dielectricsystems of the present invention is a porous dielectric material sheetwhich is positioned adjacent a resin lm sheet to function as a wick,through capillary action, to pass the dielectric liquid impregnant intothe area coextensive with the area of contact between the porousdielectric material sheet and the solid resinous dielectric materialsheet. In a resinous film dielectric, having a large amount of surfacearea, at least one such impregnation-facilitating porous layer isadvantageous. This is particularly effective, for example, in relativelylarge, tightly wound capacitors in which essentially completeimpregnation, or extremely high CSV, is required.

'Ihe porous material used is preferably kraft capacitor paper having athickness not greater than about 1.0 mil and preferably about 0.3 mil.Such paper has a dielectric strength which is relatively good ascompared to other dielectrics, although substantially less than that ofmost solid resinous materials. In addition, it has a relatively highdielectric constant which enhances the distribution of voltage in acomposite system such that a greater proportion of the voltage is placedon the higher dielectric strength resinous material. Synthetic resin orglass fiber paper may also be utilized as the wick element in thisinvention.

The effectiveness of modifying the physical characteristcs of thedielectric liquid impregnant, in order t improve impregnation and theresultant kind and degree of impregnation, has been demonstrated bytests in which capacitors having all-film dielectric spacers comprisingtwo sheets of 0.28 mil polypropylene, have been impregnated withepoxy-modiiied Pyranol 1499. Similar capacitors were impregnated with amixture of the same impregnant with another dielectric liquid, Pyranol1478, in a ratio of about 3 parts of Pyranol 1499 to 1 of the Pyranol1478. The latter impregnant is a commercially available dielectricliquid composed primarily of trichlorobenzene. While the capacitorsimpregnated with Pyranol 1499 had CSVs in the range from 400 to 1000volts A-C, the capacitors having the mixed dielectric liquid impregnantexhibited CSVs above 1500 volts A-C, indicating a substantially improveddegree of impregnation.

[While an exemplary description of this in'vention has utilizedpolypropylene as a polyolen example, the invention may also be practicedeffectively with other members of the polyolei'in family of materials,particularly polyethylene and 4-methy1 pentene-l. Representative testsindicate that these materials also may be impregnated with a dielectricliquid medium in the same manner as polypropylene, however withdifferent results. For example, high density polyethylene film wasimpregnated with Pyranol liquid dielectric by a process similar to thatdescribed with respect to polypropylene. Impregnation at temperatures inthe range of about 85 C. to 100 C. for about 16 hours was found tomarkedly increase the CSV of the composite.

Representative examples of impregnation with other dielectric liquidsnotably those previously in mixtures with Pyranol liquid, i.e., mineraloil, silicone oil, and other Pyranol liquids indicate that those liquidsmay be the total or major part of the impregnant. Other ols which 14 maybe utilized for more limited applications include cottonseed oil.]

Other combinations may include for more specialized applications[impregnated crosslinked polyethylene, or] paper materials impregnatedwith the [polyoletin] polypropylene of this invention, for example apaper impregnated with a melt or solution containing polypropylene withthe resultant material impregnated with a Pyranol dielectric.

While this invention has been disclosed with respect to particularembodiments thereof, numerous modifications may be made by those skilledin the art without departing from its true spirit and scope. Therefore,it is intended that the appended claims cover all such modifications andvariations which come within the true spirit and scope of the presentinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. An electrical capacitor assembly comprising (a) a housing;

(b) a capacitor element in said housing, said capacitor elementcomprising (l) at least a pair of electrodes, (2) a [polyolefin]kpolypropylene lm dielectric spacer between said electrodes,

(c) a dielectric liquid in said housing and essentially completelyimpregnating said [polyoletin] polypropylene film, whereby .raid film is.ruciently thin to be under a working AC electrical stress between about750 and 20,000 volts per mil thickness of the ylm at the rated voltageof the capacitor and the corona start voltage of said element beingbetween from about 750 to about 3100 volts;

(d) said dielectric liquid comprising a halogenated [aromatic compound]diphenyl having from l to 5 chlorine substituents [and from l to 3 arylgroupsl.

[2. The invention as recited in claim 1 wherein said polyolen ispolyethylene] :[3. The invention as recited in claim 1 wherein saidpolyolein is 4-methyl pentene-L] [4. An electrical capacitor assembly asrecited in claim 1 wherein said film is a polypropylene iilm.]

5. An electrical A-C capacitor assembly as recited in claim 1 whereinsaid film is a biaxially oriented polypropylene tilm.

6. An A-C electrical capacitor assembly as recited in claim 1 whereinsaid dielectric liquid comprises trichlorodiphenyl.

7. An A-C electrical capacitor assembly as recited in claim 1 whereinsaid film is a biaxially oriented polypropylene film and said dielectricliquid comprises trichlorodiphenyl.

8. The invention as recited in claim 7 wherein said dielectric liquidcomprises a mixture of trichlorodiphenyl and trichlorobenzene.

9. An electrical capacitor assembly as recited in claim 1 wherein [saidlilm is polypropylene and] said dielectric liquid comprisestrichlorodiphenyl and a stabilizer additive.

10. An electrical capacitor assembly as recited in claim 9 wherein saidstabilizer additive is an epoxide selected from the group consisting ofdipentene dioxide and lepoxyethyl-3,4-epoxycyclohexane.

11. An electrical capacitor assembly as recited in claim 6 wherein saidspacer comprises a polypropylene lm between a pair of sheets of a porousdielectric material.

12. An electrical capacitor assembly as recited in claim 6 wherein saiddielectric spacer comprises a sheet of porous dielectric materialinterposed between v[polyoleiin] polylpropylene films [at least one ofwhich is polypropylene 13. An electrical capacitor assembly as recitedin claim 6 wherein said spacer comprises a polypropylene film and atleast one sheet of a porous dielectric material adjacent thereto, andsaid lilm has one surface thereof adjacent a surface other than asurface of a porous dielectric.

14. An electrical capacitor assembly as recited in claim 6 wherein saiddielectric spacer comprises two contiguous [polyolen] polypropylenefilms, [at least one of which is polypropylene and] one of said filmshaving one surface thereof adjacent a porous dielectric material.

15. An electrical capacitor assembly as recited in claim 6 wherein saidspacer consists essentially of a pair of contiguous [polyolefin]polypropylene lms between said electrodes, [at least one of which ispolypropylene] 16. The invention as recited in claim l wherein saidcapacitor is an AC [capacitor characterized in that said film issuiciently thin to provide a working AC stress of more than about 750volts per mil thickness at its predetermined application voltage]halogenated diphenyl comprises chlorinated diphenyl.

17. An electrical capacitor assembly comprising (a) a housing;

(b) a capacitor element in said housing comprising (1) at least a pairof electrodes,

(2) a biaxially oriented polypropylene film dielectric spacer betweensaid electrodes,

(3) a paper sheet between said electrodes,

(4) said paper, film, and electrodes being in rolled form,

(c) electrical terminal means on said housing and in contact with thesaid electrodes of said capacitor element;

(d) a dielectric liquid in said housing and essentially completelyimpregnating said film and paper, whereby said lm is sufficiently thinto be under a working AC electrical stress between about 750 to 20,000volts per mil thickness o] the film at the rated voltage of thecapacitor and the corona start voltage of said element being betweenfrom about 750 to about 3100 volts;

(e) said dielectric liquid being trichlorodiphenyl with an epoxideadditive therein.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,253,199 5/1966 Cozons 317-261 2,935,668 5/1960Robinson 317-258 3,016,481 1/1962 Simpson 3l7-258 ELLIOT A. GOLDBERG,Primary Examiner U.S. C1. X.R.

